The knife switch in single or multiple blade form is one of the older and better known types of switch. Knife switches have application to many places in the railroad industry in particular. Such switches have been used in connection with individual rail cars for simultaneously connecting separated terminals. For example, a knife switch may connect both the auxiliary circuits and the propulsion circuits to power, commonly at 600 volts DC, from a third rail or pantagraph. Since the knife switch is a conductor and the terminals with which it engages are conductors, none of which are covered with insulators, it has been common to enclose the knife switch in an insulator box or enclosure when the car is in normal use. To disconnect the knife switch it is ordinarily necessary to open the cover permitting access to the knife switch and using the insulating handle to pull the switch away from the terminals.
It has been common to employ an alternative closed position for the knife switch involving just one instead of two terminals whereby the auxiliary circuits may be connected to shop power. Sometimes this arrangement involves essentially a 180.degree. rotational shift in position from the energization of both the auxiliary and the propulsion circuits to the position in which only the auxiliary circuits are employed. Commonly the switch enclosure box is too short to permit full closure of the knife switch so that the cover of the insulating housing can not be closed. This makes apparent when using power for only auxiliary circuits that the knife switch is not in position to allow power to be provided to the propulsion circuit.
There are times when it is desirable to use power from the third rail or from the pantagraph to run the auxiliary circuits without the propulsion circuits connected. For example when doing maintenance on the car in the yard on grade level track or just keeping the car in layover mode on a side track. In this case it is likewise desirable to have the cover open to make it apparent that only auxiliary circuits are energized. It is also necessary for equipment and personnel safety that it be impossible to simultaneously connect shop power to the same circuits. To do so would connect the 600 volt shop power to the 600 volt third rail. Their tolerances are quite wide and voltage sources can differ by 300 volts, thus causing arcing, equipment damage and possible maintenance operator injury.